Distortion performance of RF components subjected to broadband multichannel signal inputs are often measured by using a Multicarrier Generator (xe2x80x9cMCGxe2x80x9d) as a signal source. The measurement practice typically involves feeding the MCG""s composite signal to a Device Under Test (xe2x80x9cDUTxe2x80x9d) and observing its output signal with a spectrum analyzer in a way that permits the observation and measurement of additional spectral components that are generated due to nonlinear distortions of the DUT. Of particular importance are measurements of broadband active devices"" second and third order distortion components. These are called the Composite Second Order (xe2x80x9cCSOxe2x80x9d) and Composite Triple Beat (xe2x80x9cCTBxe2x80x9d) distortion components.
Prior art practices for measuring these distortion components using non-coherent MCG are described in detail in measurement standards adopted by the Society of Cable Telecommunications Engineers (xe2x80x9cSCTExe2x80x9d) and are available as documents entitled xe2x80x9cComposite Triple Beat Distortionxe2x80x9d, IPS-TP-206, SCTE (Oct. 31, 1997) and xe2x80x9cComposite Second Order Distortionxe2x80x9d, IPS-TP-207, SCTE (Oct. 31, 1997). These practices are designed to provide with distortion measurement methods that can closely predict actual performance of active devices in cable TV systems.
Most cable systems and some MCGs that emulate cable systems are non-coherent systems in which individual carrier frequencies are not rigidly related to each other and may each independently vary over a frequency range of hundreds or thousands of Hertz relative to their nominal frequency setting. When the carriers are unmodulated in such non-coherent systems, specific distortion components (CTB or CSO on any particular channel) constitute narrow-band signals that may each consist of hundreds or even thousands of distortion signal terms spread out in frequency over several kHz. This necessitates the setting of the Spectrum Analyzers"" Resolution Bandwidth (xe2x80x9cRBWxe2x80x9d) to 30 kHz and performing video filtering with a low video bandwidth (10 Hz or 30 Hz), and video averaging if possible.
It is important to note that both video filtering and video averaging applied in such measurements essentially amount to time-averaging of the output of the spectrum analyzer""s LOG amplifier which is fed by its IF envelope detector. Hence, the practice in the industry is to report the average of decibel values of the fluctuating distortion power rather than its average power in decibels. It can be shown mathematically that absent such time averaging (i.e. video bandwidths settings that exceed the RBW), the first order probability density function of such measured results is a Log-Rayleigh distribution and that the variance is approximately 5.6 dB, independent of the absolute levels, the channel or even the order of the distortion term.
The results under video filtering conditions depend on many factors including the spectral distribution of the distortion signals. In this context, if the distortion power spectra does indeed fall well within the 30 kHz RBW, and at the same time has a smooth spectral characteristics devoid of pronounced power variations over a frequency scale of less than the video filter bandwidth, then one can obtain a reasonably accurate and stable measurement of the average distortion power.
Typically however, the fine structure of the spectral distribution of distortion terms is unknown and may vary from one instance to another which may result in loss of both the accuracy and repeatability of the measurement. If distortion terms fall outside of the analyzer RBW setting, the analyzer will consistently underestimate the true distortion power. Alternatively, if a significant portion of the distortion power spectrum has pronounced spectral power variations over a frequency range smaller than the video filter bandwidth, then distortion measurements will not be repeatable as a consequence of insufficient video averaging of very slow fluctuations. Ironically, this phenomena of slow fluctuation in the averaged distortion power is more pronounced with improved frequency precision of the non-coherent carriers, as the distortion components are dispersed over a narrower bandwidth, giving rise to large spectral power variations over a narrower frequency range.
The slow fluctuation and lack of repeatability of these distortion measurements was recognized and prior art methods attempting to mitigate it have been reported in a conference paper entitled xe2x80x9cCTB/CSO Measurement Repeatability Improvements Using Uniformly Distributed Noncoherent Carrier Frequenciesxe2x80x9d, by E. J. McQuillen and D. Schick, published in the Proceedings of the SCTE Emerging Technologies Conference, pp 315-328; San Antonio, Jan. 28-30, (1998). These authors proposed a xe2x80x9cPseudorandom Spreadingxe2x80x9d method of intentionally dispersing the actual frequencies of all the carriers by pseudorandom frequency deviations of up to a few kHz so that the resulting distortion components would appear spread out over a frequency range that is up to three times wider than that, thereby reducing the likelihood of slow distortion envelope fluctuations.
One of the difficulties with such a xe2x80x9cPseudorandom Spreadingxe2x80x9d method is that by its very nature, it spreads out the distortion spectra away from the center of the Resolution Bandwidth Filter. The 30 kHz RBW filter mode used in the spectrum analyzer has a 3 dB bandwidth of 30 kHz, which means that a frequency response loss of 1-2 dB can easily be incurred for these dispersed distortion components. This factor can cause a systematic error by underestimating the distortion power. Indeed, the above referenced paper""s authors themselves report without any explanation a measurement bias of 2 dB as compared to the non-dispersed case. Furthermore, the actual bias depends on the specific tone that is being measured and the specific collection of terms and their respective frequency deviations from the center of the filter. Alternatively, Expanding the RBW might reduce this bias but it will be at the expense of noise immunity.
In other approaches, prior art use of coherent sources for distortion tests was also made but for the reasons discussed below was often met with significant inconsistencies and deviations from expected results. One type of a coherent MCG source differs from non-coherent head-ends and simulators in that it generates an Incrementally Related Coherent (xe2x80x9cIRCxe2x80x9d) multicarrier signal. The multicarrier signal is generated in accordance with an IRC frequency plan in which carrier frequencies fn are given by the following formula:
fn=nxc2x76 MHz+1.2625 MHz,
where n represents the carrier index. Thus, carriers are spaced by 6 MHz and fall at offsets of 1.2625 MHz relative to 6 MHz multiples. For test purposes, an MCG in which n takes on values between 9 and 135 is preferable. All carriers generated by such coherent source are locked to a common signal reference. Small deviations in the reference frequency will result in small deviations in the carrier spacing and offset. However, these deviations will be scaled for all channels with the same scale factor. Thus, all channels will still be spaced by exactly a common frequency spacing and will be located at the same fixed frequency offset relative to multiples of the carrier frequency spacing. The coherent MCG can be based, for example, on the apparatus which can generate a plurality of IRC signals with very low phase noise as described in U.S. Pat. No. 5,430,799 issued to the present inventor (hereinafter termed as the xe2x80x9c""799 Patentxe2x80x9d).
When an MCG with very low integrated phase noise is driving the DUT, the output distortion products (CTB or CSO) on a particular channel generated by the nonlinear DUT subject to the unmodulated coherent multicarrier signal are CW signals having constant amplitudes that fall exactly on the channel frequency or exactly at offsets that are integer multiples of xc2x11.2625 MHz from the channel frequency. For a particular distortion product, one can picture the hundreds or thousands of distortion terms generated by a non-coherent system converging to a single frequency term as the carrier frequency spacing between all carriers converges to a constant common value.
Reference is now made to FIG. 1 which is a captured spectrum analyzer trace. It illustrates the frequency location of distortion terms relative to the carrier frequencies in a broadband nonlinear device (which was slightly overdriven for illustration purposes). Here, the carrier of a particular test channel at the center of the scale, was turned off while all other channels are left at full power. The primary marker 10 is located at the on-channel CTB term and the delta marker 11 is on a CSO term that falls 1.2625 MHz below the channel center frequency. With a coherent MCG is used, the amplitude of the CW distortion terms discussed above are functions of the relative phases of all (coherent) distortion components, which in turn depend on the specific carrier phases of the composite multicarrier signal. It is important to note that for a given carrier phase configuration, non-fluctuating constant amplitudes of these distortion terms are only encountered if the total integrated phase noise of the carriers is very low. The direct digital synthesis technology disclosed in the ""799 Patent provides such stability based on its total integrated phase noise specification of less than one degree.
In contrast, many so called xe2x80x98coherent sourcesxe2x80x99 have been found to be frequency locked but fail to maintain rigid phase positions due to their inherent integrated phase noise that can easily produce phase fluctuations in excess of 60 or even 100 degrees (See xe2x80x9cTV Modulator Phase Noise Meaningful Performance Criteria, Specification and New Measurement Methodsxe2x80x9d by Ron D. Katznelson, NCTA Technical Conference, Atlanta; May 4, 1998). Unfortunately, these types of xe2x80x98coherentxe2x80x99 sources were the basis for much of the industry""s past experience with coherent sources, when the relative phase distribution of the carriers was never ascertained, verified or much less controlled. As a result, the use of these xe2x80x98coherentxe2x80x99 sources often produced measurement results that were less predictable and often have had significant deviations from those obtained with non-coherent sources. Therefore, it is the object of the present invention to provide for a method and an apparatus which improves the repeatability and the stability of multicarrier distortion measurements. Another object of the present invention is to provide for a method and an apparatus for accurate and unbiased measurement of distortion terms in multicarrier signal environments. Still another object of the present invention is to provide for a method and an apparatus for multicarrier distortion measurements that use reduced measurement resolution bandwidth, thereby mitigating noise contamination and improving dynamic range of such measurements.